A passive optical network (PON) is a point-to-multipoint fiber-optic network, in which unpowered optical splitters are used to enable a single optical fiber to service multiple premises, typically 16-128 homes. Reference is made to FIG. 1, which is a simplified diagram of a prior art PON 100. As seen in FIG. 1, PON 100 includes a central office node, referred to as an optical line terminal (OLT) 140, a number of user nodes, referred to as optical network units (ONUs) 120, which are near user premises 130, and fibers 150 and beam splitters 160 between them. Data is transmitted within PON 100 in packets of data frames. Downstream signals (i.e., signals transmitted from left to right in FIG. 1) originate from network services, such as an Internet service 110A, a voice over IP service 1108, a cable TV service 110C and other such services 110D. The downstream signals are broadcast to all premises 130 that share a single fiber.
PON 100 does not use electrically powered components to split the signal. Instead, the signal is distributed using beam splitters 160. Each splitter 160 typically splits the signal from a single fiber into 16, 32 or 64 fibers, and multiple splitters can be aggregated in a single cabinet. Splitter 160 does not provide switching or buffering capability. The resulting connection is called a “point-to-multipoint link”. For such a connection, ONUs 120 must perform some auxiliary functions. For example, because of the lack of switching capability, each signal leaving OLT 140 is broadcast to all ONUs 120 served by splitter 160, including ONUs 120 for which the signal is not intended. ONUs 120 filter out signals intended for other premises 130. In addition, because of the lack of buffering capability, each ONU 120 must be coordinated in a multiplexing scheme to prevent upstream signals (i.e., signals transmitted from right to left in FIG. 1) from colliding at the intersection. Two types of multiplexing are commonly used; namely, wavelength-division multiplexing and time-division multiplexing. With wavelength-division multiplexing, each ONU 120 transmits its signal using a unique wavelength. With time-division multiplexing, ONUs 120 take turns transmitting data.
For upstream signals, OLT 140 is responsible for allocating bandwidth to ONUs 120 in such a way as to provide a respective guaranteed bandwidth to each respective ONU. ONUs 120 may be located at varying distances from OLT 140 and, as such, the transmission delays for different ONUs 120 may be different. OLT 140 measures the transmission delays and sets registers to compensate for delay over all ONUs 120. Once the registers have been set, OLT 140 transmits grants to ONUs 120. A grant is permission to use a defined interval of time for upstream transmission. The grants ensure that each ONU 120 receives timely bandwidth for its service needs.
There are several drawbacks with conventional PONs. One drawback is the inability to compensate for periods of unused guaranteed bandwidth.
Another drawback relates to loss due to lack of data fragmentation. Some PONs, such as Ethernet PONs, lack data frame fragmentation and, as such, data frame sizes vary. ONUs 120 that are allocated bandwidth may not be able to use their allocated bandwidth if it is less than the bandwidth required for transmitting a currently available data frame. Such inability degrades overall performance of PON 100.
Yet another drawback relates to next generation PONs, which will operate at 10 Gbit/s rates in the near future. Current generation PONs operate at 1 Gbit/s rates, and the next generation PONs will include both 1 Gbit/s and 10 Gbit/s ONUs. Conventional bandwidth allocation methods do not take into consideration mixed 1 Gbit/s and 10 Gbit/s PONs.